Meph's main weakness is his vulnerabilty to one trick.
It's called the Moat trick - useful for ranged attackers with Teleport or Leap. Jump or teleport over blood pool to Mephisto's side, approach to make him "wake up", then go back, leap/teleport over pool again. Meph will be stuck on the other side of the blood pool and on a curtain distance completely unable to attack you. But you'll be able to attack him.
I think, this trick is easier when done on the left side of the level - councils usually spawn in the middle or right side.